Special DataBinder for data binding from web request parameters
to JavaBean objects. Designed for web environments, but not dependent on
the Servlet API; serves as base class for more specific DataBinder variants,
such as ServletRequestDataBinder.

Includes support for field markers which address a common problem with
HTML checkboxes and select options: detecting that a field was part of
the form, but did not generate a request parameter because it was empty.
A field marker allows to detect that state and reset the corresponding
bean property accordingly. Default values, for parameters that are otherwise
not present, can specify a value for the field other then empty.

Field Detail

DEFAULT_FIELD_MARKER_PREFIX

Default prefix that field marker parameters start with, followed by the field
name: e.g. "_subscribeToNewsletter" for a field "subscribeToNewsletter".

Such a marker parameter indicates that the field was visible, that is,
existed in the form that caused the submission. If no corresponding field
value parameter was found, the field will be reset. The value of the field
marker parameter does not matter in this case; an arbitrary value can be used.
This is particularly useful for HTML checkboxes and select options.

Method Detail

setFieldMarkerPrefix

Specify a prefix that can be used for parameters that mark potentially
empty fields, having "prefix + field" as name. Such a marker parameter is
checked by existence: You can send any value for it, for example "visible".
This is particularly useful for HTML checkboxes and select options.

Default is "_", for "_FIELD" parameters (e.g. "_subscribeToNewsletter").
Set this to null if you want to turn off the empty field check completely.

HTML checkboxes only send a value when they're checked, so it is not
possible to detect that a formerly checked box has just been unchecked,
at least not with standard HTML means.

One way to address this is to look for a checkbox parameter value if
you know that the checkbox has been visible in the form, resetting the
checkbox if no value found. In Spring web MVC, this typically happens
in a custom onBind implementation.

This auto-reset mechanism addresses this deficiency, provided
that a marker parameter is sent for each checkbox field, like
"_subscribeToNewsletter" for a "subscribeToNewsletter" field.
As the marker parameter is sent in any case, the data binder can
detect an empty field and automatically reset its value.

getFieldMarkerPrefix

setFieldDefaultPrefix

Specify a prefix that can be used for parameters that indicate default
value fields, having "prefix + field" as name. The value of the default
field is used when the field is not provided.

Default is "!", for "!FIELD" parameters (e.g. "!subscribeToNewsletter").
Set this to null if you want to turn off the field defaults completely.

HTML checkboxes only send a value when they're checked, so it is not
possible to detect that a formerly checked box has just been unchecked,
at least not with standard HTML means. A default field is especially
useful when a checkbox represents a non-boolean value.

The presence of a default parameter preempts the behavior of a field
marker for the given field.

getFieldDefaultPrefix

setBindEmptyMultipartFiles

Set whether to bind empty MultipartFile parameters. Default is "true".

Turn this off if you want to keep an already bound MultipartFile
when the user resubmits the form without choosing a different file.
Else, the already bound MultipartFile will be replaced by an empty
MultipartFile holder.

checkFieldMarkers

Check the given property values for field markers,
i.e. for fields that start with the field marker prefix.

The existence of a field marker indicates that the specified
field existed in the form. If the property values do not contain
a corresponding field value, the field will be considered as empty
and will be reset appropriately.